How & Why Do Leaves Change Color?

Despite their astonishing record of losses when dealing with lumberjacks and beavers, trees are pretty tough customers. Their trunks, branches, roots and twigs are all more than capable of enduring a winter's worth of freezing temperatures, snow, sleet and hail. Their leaves, though? Eh, not so tough. The broad, thin leaves of a broadleaf tree (like a maple, an oak, a birch, or a poplar) are an Achilles' heel when winter comes, and are vulnerable to freezing and damage from the elements. In order to survive, the trees either have to somehow protect the delicate leaves or shed them.

Evergreen trees—your pines, spruces, firs, etc.— went the protection route. Their leaves, or needles, are covered in a waxy coating to resist freezing, allowing them to live for years or even decades before falling off and being replaced. The leaves of deciduous trees, on the other hand, are cast off with the arrival of winter. The chemical processes that prepare them for their send-off also treat us to the season's vibrant colors.

Color Coding

Green: The green color of leaves throughout spring and summer comes from chlorophyll, a pigment vital to photosynthesis.

As we get closer to autumn and some parts of the planet get fewer hours of sunlight, trees respond by stopping the food-making photosynthesis process and slowing the production of chlorophyll until, eventually, they stop producing it altogether and the green color of the leaf fades

leaves-mapleYellow and Orange: Along with chlorophyll, there are yellow and orange pigments, carotene and xanthophyll, inside some trees' leaves. For most of the year, these pigments are masked by chlorophyll, but as the chlorophyll breaks down and the green color dissipates, the yellow to orange colors become visible.

Red: Another class of pigment that occurs in leaves is the anthocyanins. Anthocyanins, unlike carotene and xanthophyll, are not present in leaves year-round. It isn't until the chlorophyll begins breaking down that the plant begins to synthesize anthocyanin. Why do trees begin producing a different pigment in leaves they're getting ready to lose? The prevailing theory is that anthocyanins protect leaves from sun damage, lower their freezing point, allow them to remain on the tree longer, and buy the tree more time to recover nutrients from its leaves. The colors that anthocyanins produce are dependent on the pH of the leaves' cell sap. Very acidic sap results in a bright red color, while less acidic sap leads to a purplish red.

Brown: The humdrum color is the result of waste products trapped in the leaves.

That covers the basics of how each of the colors can be produced. But which color we ultimately see depends on several factors, such as"¦

Species: Certain colors are characteristic of particular tree species and can be used to help identify the type of tree you're looking at. Oak leaves turn red, brown, or russet, hickories turn golden bronze, poplars turn golden yellow, dogwoods turn a purplish red, beeches turn a light yellow/tan, birches turn bright yellow, sugar maples turn orange-red, black maples turn a glowing yellow, and red maples turn scarlet. Some trees, notably elms, don't go through much color change at all; there's just a dull brown and then the leaf is gone with the wind.

Weather: The temperature and moisture levels a tree is exposed to before and during the time its leaves' chlorophyll breaks down can affect color. Sunny days and cool nights favor anthocyanin production and bright red leaves. On cloudy days, anthocyanin isn't as chemically active and allows the orange or yellow pigments to take center stage.

Geography: Autumn leaves in Europe tend to be mostly yellow, but the US and East Asia seem to favor red leaves. Scientists from Israel and Finland recently put forth a theory about this color difference in the journal New Phytologist1. The scientists think that some 35 million years ago—amid a series of ice ages—many tree species evolved to become deciduous and produced red leaves to ward off insects. In North America and Asia, north-to-south mountain chains enabled the north and south spread of plants and animals corresponding with the advance and retreat of ice. In Europe, east-to-west mountain ranges like the Alps trapped plant and animal life. Many tree species (and the insects that depended on them) died out when the ice advanced. At the end of repeated ice ages, say the scientists, the tree species that survived didn't need red leaves to cope with the insects that were left, so they stopped producing red pigments and stuck with yellow.

The Dead Leaves and the Dirty Ground


While all this color changing and autumn magic is going on, the tree is preparing to cast off its leaves. Around the same time that chlorophyll production slows down, the veins that transport nutrients and water to the leaf from the rest of the tree get closed off. A layer of cells at the base of the leaf stem, called the separation layer, swells and forms a cork-like material, gradually severing the tissue that connects the leaf to the branch. The leaf falls off and the tree seals the cut—so when the leaf is blown off or falls from its own weight, a leaf scar is left behind.

1Lev-Yadun, S and Holopainen, J. (2009). Why red-dominated autumn leaves in America and yellow-dominated autumn leaves in Northern Europe? New Phytologist Volume 183(3): 506-512. doi:10.1111/j.1469-8137.2009.02904.x
Special thanks to Damian Dockery, who provided the foliage photos. See more of his work at

Big Questions
Are There Number 1 Pencils?

Almost every syllabus, teacher, and standardized test points to the ubiquitous No. 2 pencil, but are there other choices out there?

Of course! Pencil makers manufacture No. 1, 2, 2.5, 3, and 4 pencils—and sometimes other intermediate numbers. The higher the number, the harder the core and lighter the markings. (No. 1 pencils produce darker markings, which are sometimes preferred by people working in publishing.)

The current style of production is profiled after pencils developed in 1794 by Nicolas-Jacques Conté. Before Conté, pencil hardness varied from location to location and maker to maker. The earliest pencils were made by filling a wood shaft with raw graphite, leading to the need for a trade-wide recognized method of production.

Conté’s method involved mixing powdered graphite with finely ground clay; that mixture was shaped into a long cylinder and then baked in an oven. The proportion of clay versus graphite added to a mixture determines the hardness of the lead. Although the method may be agreed upon, the way various companies categorize and label pencils isn't.

Today, many U.S.  companies use a numbering system for general-purpose, writing pencils that specifies how hard the lead is. For graphic and artist pencils and for companies outside the U.S., systems get a little complicated, using a combination of numbers and letters known as the HB Graphite Scale.

"H" indicates hardness and "B" indicates blackness. Lowest on the scale is 9H, indicating a pencil with extremely hard lead that produces a light mark. On the opposite end of the scale, 9B represents a pencil with extremely soft lead that produces a dark mark. ("F" also indicates a pencil that sharpens to a fine point.) The middle of the scale shows the letters and numbers that correspond to everyday writing utensils: B = No. 1 pencils, HB = No. 2, F = No. 2½, H = No. 3, and 2H = No. 4 (although exact conversions depend on the brand).

So why are testing centers such sticklers about using only No. 2 pencils? They cooperate better with technology because early machines used the electrical conductivity of the lead to read the pencil marks. Early scanning-and-scoring machines couldn't detect marks made by harder pencils, so No. 3 and No. 4 pencils usually resulted in erroneous results. Softer pencils like No. 1s smudge, so they're just impractical to use. So No. 2 pencils became the industry standard.

WANG ZHAO/AFP/Getty Images
Big Questions
What Are Curlers Yelling About?
WANG ZHAO/AFP/Getty Images
WANG ZHAO/AFP/Getty Images

Curling is a sport that prides itself on civility—in fact, one of its key tenets is known as the “Spirit of Curling,” a term that illustrates the respect that the athletes have for both their own teammates and their opponents. But if you’re one of the millions of people who get absorbed by the sport once every four years, you probably noticed one quirk that is decidedly uncivilized: the yelling.

Watch any curling match and you’ll hear skips—or captains—on both sides barking and shouting as the 42-pound stone rumbles down the ice. This isn’t trash talk; it’s strategy. And, of course, curlers have their own jargon, so while their screams won’t make a whole lot of sense to the uninitiated, they could decide whether or not a team will have a spot on the podium once these Olympics are over.

For instance, when you hear a skip shouting “Whoa!” it means he or she needs their teammates to stop sweeping. Shouting “Hard!” means the others need to start sweeping faster. If that’s still not getting the job done, yelling “Hurry hard!” will likely drive the point home: pick up the intensity and sweep with downward pressure. A "Clean!" yell means put a brush on the ice but apply no pressure. This will clear the ice so the stone can glide more easily.

There's no regulation for the shouts, though—curler Erika Brown says she shouts “Right off!” and “Whoa!” to get her teammates to stop sweeping. And when it's time for the team to start sweeping, you might hear "Yes!" or "Sweep!" or "Get on it!" The actual terminology isn't as important as how the phrase is shouted. Curling is a sport predicated on feel, and it’s often the volume and urgency in the skip’s voice (and what shade of red they’re turning) that’s the most important aspect of the shouting.

If you need any more reason to make curling your favorite winter sport, once all that yelling is over and a winner is declared, it's not uncommon for both teams to go out for a round of drinks afterwards (with the winners picking up the tab, obviously). Find out how you can pick up a brush and learn the ins and outs of curling with our beginner's guide.

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